# Characterisation and analysis of surface integrity and residual stress in laser direct energy deposited 316 L alloy subject to plasticity ball burnishing

## TL;DR

Ball burnishing improves surface quality and creates compressive stress in laser-deposited 316L alloys, enhancing hardness and grain structure.

## Contribution

The study reveals directional effects of ball burnishing on surface integrity and residual stress in laser-deposited 316L alloys.

## Key findings

- Burnishing reduced surface roughness (Ra by 76%, Sa by 51%) and converted tensile to compressive stresses.
- Grain structure changed to equiaxed within 50 μm, with the most change in the plane normal to the burnishing direction.
- Microhardness increased by 32% at the surface, with a hardened layer up to 400 μm deep.

## Abstract

This paper presents the effect of ball burnishing on the surface integrity and residual stress of laser-direct energy-deposited (DED) 316 L alloys, with a particular focus on surface modification characteristics across two directional planes relative to the burnishing direction. The results show that the burnishing significantly improved surface finish, reducing Ra and Sa by 76% and 51%, respectively. Additionally, the burnishing altered the grain structure from cellular/columnar to equiaxed within 50 μm deep from the top surface, with the most pronounced changes occurring in the cross-sectional plane normal to the burnishing direction. The process also converted tensile stresses into compressive stresses, with the peak compressive stress being 99% higher than that of the ground surface. Notably, the compressive stress was higher along normal to the burnishing direction compared to the burnishing direction itself. Furthermore, the burnishing increased the full width at half maximum (FWHM) by broadening X-ray diffraction (XRD) peaks, with the greatest increase observed at a depth of 68 μm, confirming the severe grain alternations. Due to grain modification and dislocation movement, the burnishing increased microhardness by 32% at the top surface, with a hardened layer extending up to 400 μm in depth. The improvement in hardness was more significant on the plane normal to the burnishing direction.